1. Field of the Invention
This invention relates to internally implanted electronic devices adapted to be operated in a variety of modes for stimulating body tissue or to monitor various conditions of the device itself or of body tissue, e.g., the patient's heart.
2. Description of the Prior Art
Heart pacers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assigned of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted within the human body and operative in such an environment for long periods of time. Typically, such pacemakers are implanted in the pectorial region or in the abdominal region of the patient by a surgical procedure, whereby an incision is made in such region and the pacemaker with its own internal power supply, is inserted within the patient's body. This pacer operates asynchronously to provide fixed-rate stimulation not automatically changed in accordance with the body's needs, and has proven effective in alleviating the symptoms of complete heart block. An asynchronous pacer, however, has the possible disadvantage of competing with the natural, physiological pacemaker during episodes of normal sinus condition.
An artificial pacer of the demand type has been developed wherein the artificial stimuli are initiated only when required and subsequently can be eliminated when the heart returns to the sinus rhythm. Such a demand pacer is shown in U.S. Pat. No. 3,478,746 issued Nov. 18, 1969 and entitled "CARDIAC IMPLANTABLE DEMAND PACEMAKER". The demand pacer solves the problem arising in asynchronous pacers by inhibiting itself in the presence of ventricular activity (the ventricle's R wave), but by coming "on line" and filling in missed heartbeats in the absence of ventricular activity.
A problem with such prior art, implantable demand pacers is that there was no way to temporarily increase or decrease the rate or other operating parameter at which these stimulating pulses are generated without surgical intervention. Still another problem is the great difficulty in establishing the battery life remaining, in detecting and correcting a failing electrode, and in establishing an adequate R-wave sensitivity safety margin in an implanted demand pacer.
Some implantable cardiac pacers presently constructed have a rate overdrive capability but do not adequately check the viability of the demand function. Other devices are provided with a magnetic reed switch arrangement which can deactivate the demand amplifier for the purpose of checking the demand function but are lacking in a rate overdrive capability.
Another improvement which has occurred since Greatbatch first disclosed the implantable cardiac pacemaker is means to allow the pacemaker to be reprogrammed after it has been implanted. In U.S. Pat. No. 3,805,796 in the name of Reese Terry, Jr. et al, entitled "Implantable Cardiac Pacer having Adjustable Operating Parameters", which issued in 1974, circuitry is disclosed to allow the rate of the pacemaker to be noninvasively changed after it has been implanted. The rate varies in response to the number of times a magnetically operable reed switch is closed. The Terry et al device operates by counting the number of times the reed switch is closed and storing that count in a binary counter. Each stage of the counter is connected to either engage or bypass one resistor in a serially connected resistor chain, which chain is a part of the RC time constant controlling the pacemaker rate.
The concept of the Terry et al device has been improved upon by the apparatus shown in U.S. Pat. No. 4,066,086 in the name of John M. Adams et al, entitled "Programmable Body Stimulator", which issued in 1978, and which discloses a programmable cardiac pacemaker that responds to the application of radio frequency (RF) pulse bursts while a magnetic field held in close proximity to a magnetically operated reed switch included within the pacemaker package holds the reed switch closed. In the Adams et al circuit, again only the rate is programmable in response to the number of RF pulse bursts applied. The use of radio frequency signals to program cardiac pacemakers was earlier disclosed by Wingrove in the U.S. Pat. No. 3,833,005 entitled "Compared Count Digitally Controlled Pacemaker" which issued in 1974. The Wingrove device was capable of having both the rate and pulse width programmed. However, no pacemaker has ever been described which is capable of having more than two parameters programmed or selected features or tests programmed on command. Such a pacemaker could be called a universally programmable pacemaker.
One area where cardiac pacing technology has lagged behind conventional state of electronic technology involves utilization of digital electrical circuits. One reason for this has been the high energy required to operate digital circuits. However, with more recent technology advances in complementary metal oxide semiconductor (CMOS) devices fabricated on large scale integrated circuits, together with the improvements of cardiac pacemaker batteries, digital electronic circuits are beginning to be utilized in commercial pacemakers. The inherent advantages of digital circuits are their accuracy, and reliability. Typically, the digital circuit is operated in response to a crystal oscillator which provides a very stable frequency over extended periods of time. There have been suggestions in the prior art for utilizing digital techniques in cardiac stimulators and pacemakers since at least 1966. For instance, see the article by Leo F. Walsh and Emil Moore, entitled "Digital Timing Unit for Programming Biological Stimulators" in The American Journal of Medical Electronics,First Quarter, 1977, Pages 29 through 34. The first patent suggesting digital techniques is U.S. Pat. No. 3,557,796 in the name of John W. Keller, Jr., et al, and is entitled "Digital Counter Driven Pacer", which issued in 1971. This patent discloses an oscillator driving a binary counter. When the counter reaches a certain count, a signal is provided which causes a cardiac stimulator pulse to be provided. At the same time the counter is reset and again begins counting the oscillator pulses. Additionally, in the Keller et al patent, there is disclosed the digital demand concept, in which the counter is reset upon the sensing of a natural heartbeat, and the digital refractory concept, in which the output is inhibited for any certain time after the provision of a cardiac stimulating pulse or the sensing of a natural beat.
As mentioned above, digital programming techniques are shown in both the Terry et al U.S. Pat. No. 3,805,796 and the Wingrove U.S. Pat. No. 3,833,005. Wingrove additionally discloses digital control circuitry for controlling the rate of the stimulating pulses by providing a resettable counter to continually count up to a certain value that is compared against a value programmed into a storage register. The Wingrove patent also shows provisions for adjusting the output pulse width by switching the resistance in the RC circuit which controls the pulse width.
Other patents disclosing digital techniques useful in cardiac pacing include U.S. Pat. Nos. 3,631,860 in the name of Michael Lopin entitled "Variable Rate Pacemaker, Counter-Controlled, Variable Rate Pacer"; 3,857,399 in the name of Fred Zacouto entitled "Heart Pacer"; 3,865,119 in the name of Bengt Svensson and Gunnar Wallin entitled "Heartbeat Accentuated with Controlled Pulse Amplitude"; 3,870,050 in the name of Wilson Greatbatch entitled "Demand Pacer"; 4,038,991 in the name of Robert A. Walters entitled "Cardiac Pacer with Rate Limiting Means"; 4,043,347 in the name of Alexis M. Renirie entitled "Multiple-Function Demand Pacer with Low Current Drain"; 4,049,003 in the name of Robert A. Walters et al entitled "Digital Cardiac Pacer"; and 4,049,004 in the name of Robert A. Walters entitled "Implantable Digital Cardiac Pacer Having Externally Selectable Operating Parameters and One Shot Digital Pulse Generator for Use Therein".
Though there has been suggested that various parameters, i.e., pulse width and rate, may be changed within a internally implanted pacer, it is desired to provide a device that is capable of operating in various, different pacing and/or sensing modes. The systems of the prior art are capable of storing by means of digital counter circuitry a programmable word indicative of desired rate or pulse width. In an internally implanted device, the space to incorporate a plurality of such counters whereby a number of such functions could be programmed, is indeed limited. Further, there are considerations of the available energy to energize such counters, as well as of the life of its internal power source as a result of the imposed drain. It is well recognized in the art that the complexity of the circuit incorporated within an internally implanted device is limited by many factors including the drain imposed upon the battery and therefore the expected life of a battery before a surgical procedure is required to replace the device's power source, e.g., a battery.